US20160162776A1 - Systems and methods for synchronizing a plurality of rfid interrogators in a theatre of operation - Google Patents

Abstract

RFID tags are used for many purpose including tracking. RFID interrogators are used to retrieve information from tags. In many applications, a plurality of RFID interrogators are required. Synchronization between interrogators in the same theatre of operation is critical to ensure that their broadcasts do not interfere with each other. In fixed RFID interrogator applications, RFID interrogators can be wired together allowing a channel to synchronize the transmissions of the RFID interrogators. Methods described herein can ensure that synchronization is maintained in the event of the failure of a synchronizing master. Furthermore, additional methods for synchronizing RFID interrogators in wireless applications are described allowing synchronization in the absence of wired connections between interrogators.

Description

RELATED APPLICATIONS INFORMATION
• This present application is a continuation of U.S. patent application Ser. No. 14/225,359, filed on Mar. 25, 2014, entitled “Systems And Methods For Synchronizing A Plurality Of RFID Interrogators in A Theatre Of Operation” which claims the benefit of priority as a Continuation under 35 U.S.C. §120 of U.S. patent application Ser. No. 11/766,749, filed Jun. 21, 2007, entitled “Systems And Methods For Synchronizing A Plurality Of RFID Interrogators In A Theatre Of Operation,” which in turn claims the benefit of priority under 35 U.S.C. 119(e) to Provisional Patent Application Ser. No. 60/805,423, entitled “RFID Smart Cabinet And A Multidocument Read/Write Station,” filed Jun. 21, 2006, all of which are incorporated herein by reference as if set forth in full.
BACKGROUND
• 1. Field of the Invention
• The field of the invention relates generally to Radio Frequency Identification (RFID) systems and more particularly to systems and methods for synchronizing a plurality of RFID interrogators in a theatre of operation.
• 2. Background of the Invention
• FIG. 1 illustrates abasic RFID system100. Abasic RFID system100 comprises three components: an antenna orcoil104, aninterrogator102 withdecoder112, and a transponder, orRF tag106 which is often electronically programmed with unique information.Antenna104 emitsradio signals110 to activate and read and write data to tag106.Antenna104 is the conduit betweentag106 andinterrogator102, which controls data acquisition and communication.Antennas104 are available in a variety of shapes and size, for example, in certain embodiments they can be built into a door frame to receive tag data from persons or things passing through the door. In other embodiments,antennas104 can, for example, be mounted on an interstate toll booth to monitor traffic passing by on a freeway. Further, depending on the embodiments, the electromagnetic field, i.e.,radio signal110, produced by anantenna104 can be constantly present when, e.g.,multiple tags106 are expected continually. if constant interrogation is not required, thenradio signal110 can, for example, be activated by a sensor device.
• Oftenantenna104 is packaged withinterrogator102. Aconventional interrogator102 can emitradio signals110 in ranges of anywhere from one inch to 100 feet or more, depending upon the power output and the radio frequency used. When anRFID tag106 passes through an electromagnetic zone associated withradio signal106, it detectsradio signal106, which can comprise an activation signal. In some embodiments, interrogators can comprise multiple antenna, though typically only one transmits at a time.
• RFD tags106 come in a wide variety of shapes and sizes. Animal tracking tags, for example, inserted beneath the skin of an animal, can be as small as a pencil lead in diameter and one-half inch in length.Tags106 can be screw-shaped for insertion, e.g., in order to identify trees or wooden items, or credit-card shaped for use in access applications. Anti-theft hard plastic tags that includeRFID tags106 can be attached to merchandise in stores. Heavy-duty RFD tags can be used to track intermodal containers, heavy machinery, trucks, and/or railroad cars for maintenance and/or tracking purposes. A multitude of other uses and applications also exists, and many more will come into being in the future.
• RFID tags106 are categorized as either active or passive.Active RFID tags106 are powered by an internal battery at and are typically read/write, i.e., tag data can be rewritten and/or modified. An active tag's memory size varies according to application requirements. For example, some systems operate with up to 1 MB of memory. In a typical read/write RFID work-in-process system, atag106 might give a machine a set of instructions, and the machine would then report its performance to tag106. This encoded data would then become part of the tagged part's history. The battery-supplied power of anactive tag106 generally gives it a longer read and write range. The trade off is greater size, greater cost, and a limited operational life.
• Passive RFID tags106 operate without a separate external power source and obtain operating power generated fromradio signal110.Passive tags106 are consequently much lighter thanactive tags106, less expensive, and offer a virtually unlimited operational lifetime. The trade off is that they have shorter read ranges thanactive tags106 and require a higher-poweredinterrogator102. Read-only tags are typically passive and are programmed with a unique set of data, usually 32 to 128 bits, that cannot be modified. Read-onlytags106 often operate as a license plate into a database, in the same way as linear barcodes reference a database containing modifiable product-specific information. Not allpassive tags106 are read-only tags.
• RFID systems are also distinguishable by their frequency ranges. Low-frequency, e.g., 30 KHz to 500 KHz, systems have short reading ranges and lower system costs. They are commonly used in security access, asset tracking, and animal identification applications. High-frequency, e.g., 850 MHz to 950 MHz and 2.4 GHz to 2.5 GHz, systems offer long read ranges, e.g., greater than 90 feet, high reading speeds, and are used for such applications as railroad car tracking and automated toll collection, however, the higher performance of high-frequency RFID systems100 incurs higher system costs.
• The significant advantage of all types ofRFID systems100 is the noncontact, non-line-of-sight nature of the technology.Tags106 can be read through a variety of substances such as snow, fog, ice, paint, crusted grime, and other visually and environmentally challenging conditions, where barcodes or other optically read technologies cannot typically be used.RFID tags106 can also be read in challenging circumstances at high speeds, often responding in less than 100 milliseconds. RFID has become indispensable for a wide range of automated data collection and identification applications that would not be possible otherwise.
• Aconventional RFID interrogator102 comprises anRF transceiver106 and adecoder112.Decoder112 can, for example, be a micro controller or other processing circuit configured to carryout the required functions. Often,decoder112 is interfaced withmemory114. Firmware instructions used bydecoder112 to control the operation ofinterrogator102 can be stored inmemory114, along with RFID instructions that can be communicated toRFID tag106 and can be used to control acquisition of information fromRFID tags106.Memory114 can, depending on the embodiment, comprise one or more memory circuits.
• FIG. 2 shows an example transmission operation of an RFID interrogator.Graph200 shows a transmission of the RFID interrogator when no data is transmitted. At the start of eachframe202,interrogator102 can be configured to transmitframe synchronization pulses204, which can have a much shorter width than the period associated withframe202.RFID interrogator102 can transmit data toRFID tag106 by modifying the frame synchronization pulses, for instance by doubling the pulses to represent a binary “zero” and tripling the synchronization pulses to represent a binary “one.”Graph220 shows an example of such a transmission method by an RFID interrogator.Double pulses222 and230, which comprise two pulses sent within a short period compared to the frame period; represent the transmission of a “zero,”Triple pulse226, which comprise three pulses sent within a short period compared to the frame period, represent the transmission of a “one.” Remainingsingle pulses224 and228 do not represent data and synchronize the associated frames.
• Another method of modifying frame synchronization pulses used by RFID interrogators is to use wider pulses to represent a “zero” and still wider pulses to represent a “one.”Graph240 shows an example of such a transmission method. The “wider”pulses242 and250, which are still short compared to the frame period, represent the transmission of a “zero.” The “widest” pulse246, which is still short compared to the frame period but wider thanpulses242 and250, represent the transmission of a “one.” The remaining “normal”width pulses244 and248 do not represent data and synchronize the associated frames202.
• Graphs220 and240 illustrate just two possible examples of communication protocols that can be used to facilitate transmission of data insystem100.
• In response to interrogation signals from theinterrogator102, RFID tags106 can be configured to respond in the second half offrames202 Furthermore, in many embodiments of anRFID interrogation systems100 bothtags106 andinterrogator102 operate in the same frequency range. The synchronization pulses, whether “normal” or modified to carry data can serve two additional purposes. First, the pulses can be used to define the boundaries offrames202 so thetags106 can respond at the appropriate time. Second, the pulses supply power for passive RFID tags106.
• FIG. 3 depicts aninterrogation theatre300 comprising a plurality of interrogators, of whichinterrogators310 and340 are shown for illustrative purposes. In addition,theatre300 comprises a plurality of tags, of which tags320,322, and344 are shown for illustrative purposes.Tags320,322, and344 can for example, be similar to, or the same as,tag106 described above. If allowed to operate independently, these readers can severely interfere with each other. To illustrate, inFIG. 3, RFID tags320 and322 arenear interrogator310, whileRFID tag344 isnear interrogator340. Temporally,interrogator310 has just transmitted its request through itsantenna312 and is now awaiting a response signal from any nearby RFID tags. Because RFID tags320 and322 are near to interrogator310, they respond withRF signals330 and332, respectively; however, at approximately the same time,interrogator340 wishes to interrogate RFID tags nearby such asRFID tag344, by transmittingsignal346 throughantenna342. Since theresponses330 and332 are on the same frequency as theinterrogation signal346, andinterrogation signal346 can be of greater power thansignals330 and332,interrogator310 may only detect the signal frominterrogator340 rather than fromRFID tags320 and322.
• FIG. 4 depicts the timing of the example given above.Graph400 depictsinterrogator310 attempting to interrogate nearby RFID tags using the communications protocol illustrated bygraph220.RFID tag320 responds and itsRF output signal330 is graphed over time ingraph410; however, with anunsynchronized RFID interrogator340 also attempting to interrogate nearby RFID tags as depicted ingraph420, associatedsignal346 can interfere withsignal330. As a result,antenna312 sees the signal depicted ingraph430, where rather than seeingpulses412 and414 of signal330 (graph410),interrogator310 is likely to see something likepulses432 and436 dominated by the influence of signal346 (graph420) ofinterrogator340. As a result,interrogator310 may interpretpulses434 and438 ofinterrogator340 as coming fromRFID tag320, orinterrogator310 may just fail to code any signal or may receive corrupted information.
SUMMARY
• An RFID system comprises a plurality of synchronized RFID interrogators. Synchronization between interrogators in the same theatre of operation can be critical to ensure that their broadcasts do not interfere with each other. In fixed RFD interrogator applications, RFID interrogators can be wired together to allow synchronization of transmissions of the RFID interrogators.
• In one aspect, synchronization is maintained in the event of the failure of a synchronizing master.
• In another aspect, synchronizing RFID interrogators in the absence of wired connections between interrogators is provided.
• These and other features, aspects, and embodiments of the invention are described below in the section entitled “Detailed Description.”
BRIEF DESCRIPTION OF THE DRAWINGS
• Features, aspects, and embodiments of the inventions are described in conjunction with the attached drawings, in which:
• FIG. 1 is a diagram illustrating anexemplary RFID system100;
• FIG. 2 is a diagram illustrating example transmission protocols that can be used in the system ofFIG. 1;
• FIG. 3 is a diagram illustrating an exemplary interrogation theatre comprising a plurality of interrogators;
• FIG. 4 is a diagram illustrating example signals and timing for the theatre ofFIG. 3;
• FIG. 5 is a diagram illustrating an example baggage tracking system that includes a plurality of interrogators synchronized in accordance with one embodiment;
• FIG. 6 is a diagram illustrating example signals and timing for the system ofFIG. 5;
• FIG. 7 is a diagram illustrating a temporal overview of a self-promotion process for synchronized interrogators in accordance with one embodiment;
• FIG. 8 is a flowchart illustrating an example method for interrogator promotion in accordance with one embodiment;
• FIG. 9 is a flowchart illustrating an example method for adjusting frame synchronization pulses, when interference is detected in accordance with one embodiment;
• FIG. 10 is a diagram illustrating an example embodiment of RFID tag response encoding in accordance with one embodiment; and
• FIG. 11 is a diagram illustrating an example detecting interference in accordance with one embodiment.
DETAILED DESCRIPTION
• In one embodiment,synchronization signal512 supplied by synchronization master510 can be used byinterrogators520,530, and540 to ensure that the corresponding interrogator signals524,534, and544 do not interfere with reception of signals transmitted by RFID tags550-564. For example, ifgraphs620,630, and640 correspond tosignals544,534, and524, respectively, then it can be seen that synchronization signal512 (graph610) can cause each interrogator to begin transmission at the start of a common frame period. In other words,interrogators520,530, and540 can be configured such that each interrogator upon receipt of a pulse insignal512. This can ensure that each interrogator is finished transmitting before the start of the second half offrame604, devoted by dashedline606 during which responses from RFID tags550-564 we received. Thus, interference signals524,534, and544 with those transmitted from RFID tags550-564 can be avoided.
• As mentioned above, thestart602 offrames604, depending on the requirements of a particular implementation, begin some fixed period (Δd) after the rising edge of thepulses comprising signal512 as illustrated ongraph610. The delay (Δd) can, for example, be long enough to account for various delays associated with thecircuitry comprising interrogators520,530, and540.
• In order to avoid the problem illustrated inFIGS. 3 and 4, for example, interrogators in a theatre ofoperation300 can be synchronized as described herein.FIGS. 5 and 6 illustrate an embodiment of asystem500 with multiple interrogators in a single theatre of operation. In one embodiment, for example, such a system can be employed in a baggage tracking system, e.g., at an airport.
• FIG. 5 depicts abaggage tracking system500 where a plurality ofinterrogators520,530 and540 are synchronized in accordance with the systems and methods described herein. In airportbaggage tracking system500, the objective is to track the time and identity of each bag that passes by various checkpoints. To facilitate this objective, an RFID interrogator is placed at each checkpoint. Each bag is equipped with a baggage tag comprising an RFID tag. Upon the check-in, each bag is placed on some sort of conveyance mechanism, such as a conveyor belt. RFID tags550,552,554,556,558,560,562 and564 represent the RFID tags embedded in the baggage tags affixed on each bag. Each bag traverses the checkpoint monitored byinterrogator540, then the checkpoint monitored by interrogator530, followed by the checkpoint monitored byinterrogator520.
• Interrogators520,530, and540 are coupled together and to a synchronization master510, which is responsible for synchronizing the interrogators. In this embodiment, the coupling is accomplished throughwiring514. As depicted inFIG. 5, synchronization master510 can be a simple pulse generator; however, in other embodiments one ofinterrogators520,530, and540 can serve as a synchronization master. The master transmits, e.g., master510 can be configured to transmit apulse train512 to each ofinterrogators520,530 and540.Interrogators520,530, and540 can be configured, upon receiving the synchronization pulse, to transmit through antennas,522,532, and542, respectively, aradio signal524,534, and544, respectively, to interrogate passing RFID tags550-564.Signals524,534, and544 can be synchronization pulses or can carry information, e.g., using the exemplary communication protocols illustrated inFIG. 2.
• FIG. 6 illustrates an example of the signals and synchronization pulses transmitted byinterrogators520,530, and540.Graph610 depictssynchronization pulse train512.Graph620,630, and640 depict the signal outputs ofinterrogators520,530, and540, respectively. In some implementations, thestart602 of the RF frames604 do not correspond precisely with the leading edges of the pulses ingraph610, because there can be some propagation delay in the circuitry associated withinterrogators520,530, and540. A certain amount of inconsistency in the delay can be tolerated, because responses to interrogation signals are expected in the second half of the frame. As explained in detail below, each interrogator can transmit different signals without interfering with other interrogator's ability to receive RFID tag responses because regardless of the type of signal, all transmissions by all interrogators are concluded by the start of the second half of theframe604 as illustrated by dashedlines606.
• Thus,RFID interrogators520,530, and540 can be coupled to a synchronization master510 configured to synchronize transmissions from the interrogators; however, in the event of a failure associated with synchronization master510,system500 can lose its ability to synchronize the signals ofinterrogators520,530, and540. In one embodiment, this is avoided by enabling one of the remaining interrogators to become the synchronization master. Accordingly, when employing such a cooperative strategy, an interrogator can be in one of two states a synchronization master or a synchronization slave. Thus, one or more of the interrogators in a system configured to implement such a cooperative strategy must be able to both send and receive a synchronization signal.
• FIG. 7 is a diagram illustrating a temporal view of signals generated in a system employing such a cooperative strategy. In this example, each interrogator in the system is capable of both sending and receiving an interrogator signal. The system begins with an interrogator, or alternatively a signal generator, as a synchronization master configured to generate a synchronization signal as described above and illustrated in graph710. In this particular embodiment, there are three slave interrogators whose radio frame synchronization signals are depicted ingraphs720,730 and740 and whose synchronization signals are depicted in graphs725,735, and745. At750, the synchronization master suffers a failure and ceases to generate the synchronization signal.
• If just one of the interrogators in the system is capable of taking over as master, which is possible depending on the embodiment, then that interrogator will be promoted to master upon detecting the failure of the original synchronization master.
• Such configurations can be sufficient to avoid synchronization failures; however, a potential drawback to such configurations is that there is no mechanism to ensure synchronization should one promoted interrogator subsequently fail, fails to generate of synchronization signal, or fails to be promoted. Thus, it can be preferable, depending on the implementation, for a plurability of interrogators to be capable of promotion to master. In such embodiments, there must be some mechanism for determining which interrogator will become the next master.
• In one embodiment, the interrogators do not recognize an outage until a predetermined period of time has expired at752. From there each interrogator selects a random period of time to wait before it attempts to become the new synchronization master. Here, the first interrogator selects the interval between752 and754. The second interrogator selects the interval between752 and756, which happens to be a longer interval. The third interrogator happens to randomly pick the interval between752 and754, the same as the first interrogator. These wait intervals should be large compared to the frame period. In another embodiment, each interrogator at some point in its normal process can select a random time out period before registering a failure of the master. To use the same example, the period would be that between750 and754 for the first interrogator, between750 and756 for the second interrogator, and between750 and754 for the third interrogator.
• Each interrogator can be configured to send a pulse after the associated wait period to the other interrogators indicating its attempt to become the master. The other interrogators, upon receiving the pulse, can be configured to remain slaves. The new synchronization master can then send its synchronization signal to the other interrogators. A conflict can arise in the unlikely event that two or more interrogators pulse at the same time, which would be the case in the example above. In other embodiments, various schemes can be used to avoid such conflicts, or contentions. For example, in some embodiments, collision avoidance schemes can be used. Factors such as skew in the docks of each interrogator can eventually lead to a dispersion of the pulses generated. At this point, one of the interrogators will be seen as pulsing first relative to the others. This interrogator will then become the master and the others demoted to being slave interrogators. For example, at time758, the pulse, generated by the third interrogator begins to trail those of the first interrogator. The third interrogator can he configured to detect that it is no longer the master, and cease to generate synchronization pulses at time760.
• FIG. 8 illustrates a flowchart illustrating an example method for interrogator promotion in accordance with the systems and methods. Waitstates810 and850 represent the general waiting states for an interrogator in the slave state and in the master states, respectively. For example, most interrogators start inwait state810. They can transition out ofwait state810 to step812 if either a synchronization pulse is received from another interrogator or a predetermined period of time has elapsed since a synchronization pulse from a master was expected. This predetermined period is typically much larger than the frame period. If a synchronization pulse is detected, the interrogator remains a slave and can perform its regular duties by sending either a frame synchronization pulse or data to an RFID tag atstep820, and if appropriate, it can listen for RFID tag responses atstep822. Upon completion of the frame, the interrogator returns to waitstate810. On the other hand, if a synchronization pulse from a master has not been detected, atstep814, a timeout interval is selected, e.g., randomly generated as described above, the timeout interval can be with a predetermined range, which is typically many times the frame period. The interrogator then waits atstep816 for either this timeout period to expire or for a synchronization pulse from another interrogator.
• If a synchronization pulse is received atstep818, the interrogator remains a slave and can continue to perform its regular duties starting atstep820; however, if the timeout expires then the interrogator attempts to become a master atstep854 by transmitting a synchronization pulse to all the other interrogators. It then can continue to perform its regular duties by sending either a frame synchronization pulse or data to an RFID tag atstep856 and then if appropriate, it can listen for RFID tag responses atstep858. Upon completion of its regular duties, the interrogator returns to waitstate850. Inwait state850, the interrogator waits for either the start of the next frame, which is one frame period after it sent the last synchronization pulse to the other interrogators, or for a synchronization pulse from another interrogator.
• Instep852, if the interrogator detects a start of frame, it transmits a synchronization pulse to the other interrogators instep854 and the process repeats as before. But if the interrogator detects another synchronization from another interrogator, it ceases to be, a master, becomes a slave, and resumes slave duties atstep820. This can occur, for example, where the original master whose failure initiated the promotion from slave to master of steps814-854 comes back online. This can also occur if during the promotion from slave to master one or more other interrogators waited the same random interval and were simultaneously promoted to master and over time, the internal clocks of the interrogators are skewed resulting in slight deviations in the pulse interval.
• Though extremely unlikely, there may be a situation where three or more interrogators claim to be masters. Thus, in certain embodiments, each interrogator can be configured to determine under such circumstances that one of the other interrogators is the rightful master, which will cause each interrogator to switch to a slave state. At this point, no synchronization pulses are sent by any interrogator and the process for each interrogator follows the diagram inFIG. 8 by traversingsteps812,814,816 and818. At which point, a new master is selected. Alternatively, skewing that results from differences in the tolerances and errors associated with the circuitry of each interrogator can be relied on to eventually result in one interrogator being promoted over the others as described above. Obviously, the more interrogators involved the longer such a process will take. Therefore, some alternatives as described above that reduces the delay involved can be preferable for selecting among three or more contending master interrogators.
• It should be noted that in another embodiment, a random predetermined timeout greater than the predetermined “master timeout” and less than the sum of the “master timeout” and the “random timeout” range could be used inwait state810, thereby combiningsteps812,814,816, and818 into a single branch point where the detection of a synchronization pulse transitions the interrogator to step820 and the expiration of this new predetermined timeout promotes the interrogator to a master state by transitioning to step854. Such a hybrid timeout period can be used, for example, when an interrogator changes master-slave state, when a new frame is detected, when the tenth new frame is detected, etc.
• Though the above embodiments address the synchronization issues relating to the operation of multiple interrogators in a single theatre of operation, there are many applications where the wired approach described in the preceding examples is not feasible, e.g., where the RFID interrogators are mobile such as those mounted on a forklift in a warehouse tracking application, or those used as hand-held scanners in a shipment tracking application. Accordingly, one or more wireless communication links can be used to achieve synchronization. Any such wireless approach should provide an inefficient use of power and spectrum associated with the wireless communication channel or link. For instance, ideally a master interrogator should be as centrally located as possible; however, in mobile applications, the interrogators can move around in the theatre of operation. This can lead to inefficient use of power and spectrum since a master interrogator needs to generate sufficient power to be detected by even the most remote interrogator in the theatre of operation. But since it is an objective to mitigate interference between nearby interrogators, synchronization need only be enforced when interrogators are close enough to cause interference. Thus, in certain embodiments, interrogator synchronization is only employed when interference from other interrogators is detected.
• FIG. 9 is a flowchart illustrating an example method for adjusting frame synchronization pulses, when interference is detected, in accordance with the systems and methods described herein. Instep910, the interrogator waits for the start of frame. Instep912, the interrogator transmits its frame synchronization or data to nearby RFID tags at the start of the frame. The interrogator then waits, instep914, for the second half of the frame. Atstep916, the interrogator can attempt to detect any interference from other interrogators, while listening for RFID tag transmissions. If interference is detected at918, the interrogator delays, atstep920, its start of next frame time to coincide with the start of frame it detected from another interrogator at918. If no interference is detected, the interrogator processes any RFID tag transmissions it may have received atstep922. The process then repeats.
• Basically, if two interrogators come close enough to interfere, the interrogator which is first to detect interference adjusts its frame synchronization timing to match the other interrogator. Because, they are out of sync, one interrogator will have to be first in detecting interference. The environment can become much more complicated if more than two interrogators are out of frame synchronization, but realistically that is unlikely, since the frame periods are typically on the order of microseconds and physical movements take a much longer time, so by the time a third interrogator is out of sync with the first two, those two should have synchronized.
• There are many methods of distinguishing interrogation interference with RFID tag transmissions. Most of these methods involve incorporating certain patterns in the transmission protocol.
• FIG. 10 illustrates a specific embodiment of such an encoding.Graph1010 shows an interrogator's frame synchronization pulses. An RFID tag can transmit a “one” by sending a pulse in response in the second half of a first frame and no pulse in a second frame as depicted ingraph1030 and a “zero” by sending a no pulse in a first frame and a pulse in the second half of a second frame as depicted ingraph1040. In the event of no pulse as ingraph1020, there is no response from an RFID tag. In the event of a pulse in the second half frame of both a first and second frame, as ingraph1050, interference from another RFID interrogator can be deduced. More complex patterns in RFID tags transmissions can be implemented, but often these complexities lead to many more false readings.
• FIG. 11 is a flowchart illustrating an example method for synchronization that can alleviate some of the confusion associated with detecting interference. In the example ofFIG. 11, interrogators only attempt synchronization when not expecting RFID tag transmissions. In a practical system, RFID interrogators spend much of their time sending frame synchronization pulses, but not expecting a return transmission. Instep1110, the interrogator waits for its internal clock to indicate a start of frame. Instep1112, the interrogator decides if it has a pending transaction with an RFID tag, if so, atstep1114, it decides whether it is waiting on a random count due to the detection of interference from a previous iteration. If so or if there is no pending transaction, the interrogator transmits a frame synchronization pulse atstep1116. Otherwise, if there is a pending transaction and the interrogator is not waiting a random count or that count has expired hence no longer waiting, it transmits its data atstep1118. The interrogator then waits for the second half of the frame at1120. Atstep1122, the interrogator can attempt to detect any interference from other interrogators, while listening for RFID to transmissions.
• If interference is detected at1122, the interrogator behaves differently depending on whether it is expecting data from an RFID tag. If it is not expecting data atstep1124, the interrogator delays, atstep1126, its start of next frame time to coincide with the start of frame it detected from the interfering interrogator at1122. If it is expecting data, the interrogator selects a random number of frames to wait instep1128. If no interference is detected, the interrogator processes any RFID tag transmissions it may have received atstep1130. The process then repeats. In the event multiple interrogators are attempting to interrogate at the same time, the random count gives an interval when none of the interrogators are expecting data to synchronize their respective frame synchronization pulses.
• While certain embodiments of the inventions have been described above, it will be understood that the embodiments described are by way of example only. Accordingly, the inventions should not be limited based on the described embodiments. Rather, the scope of the inventions described herein should only be limited in light of the claims that follow when taken in conjunction with the above description and accompanying drawings.

Claims (20)

1. A radio frequency identification (RFID) interrogation system, comprising:

multiple RFID interrogators, wherein each of the multiple RFID interrogators is configured to transmit a plurality of synchronization pulses to surrounding RFID tags, and wherein the multiple RFID interrogators includes a first RFID interrogator, which is configured to:

transmit a synchronization pulse to nearby RFID tags;

while listening for RFID tag transmissions in response to the synchronization pulse, attempt to detect any interference from another RFID interrogator among the multiple RFID interrogators; and

if an interference is detected from a second RFID interrogator among the multiple RFID interrogators, determine if data is expected from a nearby RFID tag, and

if no data is expected, synchronize a transmission of the next synchronization pulse with the next synchronization pulse of the second RFID interrogator;

otherwise, wait for the expected data from the nearby RFID tag.

2. The RFID interrogation system ofclaim 1, wherein while detecting interference from another RFID interrogator, the first RFID interrogator is further configured to:

wait for the second half of the synchronization pulse;

if a pulse is detected in the second half of the synchronization pulse, determine if another pulse was detected in the second half of a synchronization pulse transmitted by the first RFID interrogator immediately before the synchronization pulse; and

if so, determine that an interference with the second RFID interrogator is detected.

3. The RFID interrogation system ofclaim 1, wherein if no interference is detected, the first RFID interrogator is configured to process any RFID tag transmission received during the second half of the synchronization pulse.

4. The RFID interrogation system ofclaim 1, wherein while synchronizing the transmission of the next synchronization pulse, the first RFID interrogator is configured to delay the transmission of the next synchronization pulse so that the start of the next synchronization pulse of the first RFID interrogator coincides with the start of the next synchronization pulse of the second RFID interrogator.

5. The RFID interrogation system ofclaim 1, wherein while waiting for the expected data from the nearby RFID tag, the first RFID interrogator is configured to wait a random number of synchronization pulse periods.

6. The RFID interrogation system ofclaim 1, wherein the first and the second RFID interrogators are wireless coupled to each other.

7. The RFID interrogation system ofclaim 1, wherein the second RFID interrogator moves into a wireless detection range of the first RFID interrogator.

8. The RFID interrogation system ofclaim 1, wherein at least one of the first RFID interrogator and the second RFID interrogator is a mobile RFID interrogator.

9. The RFID interrogation system ofclaim 1, wherein the first RFID interrogator is a mobile RFID interrogator mounted on a forklift for warehouse tracking.

10. The RFID interrogation system ofclaim 1, wherein the first RFID interrogator is configured in form of a hand-held scanner for shipment tracking.

11. A method for operating a first radio frequency identification (RFID) interrogator among multiple RFID interrogators while avoiding interferences with other RFID interrogators among the multiple RFID interrogators, comprising:

transmitting a synchronization pulse by the first RFID interrogator to surrounding RFID tags;

while listening for RFID tag transmissions in response to the synchronization pulse, attempting to detect any interference from another RFID interrogator among the multiple RFID interrogators;

if an interference is detected from a second RFID interrogator among the multiple RFID interrogators, determining if data is expected from a nearby RFID tag, and

if no data is expected, synchronizing a transmission of the next synchronization pulse of the first RFID interrogator with the next synchronization pulse of the second RFID interrogator;

otherwise, waiting for the expected data from the nearby RFID tag.

12. The method ofclaim 11, wherein detecting interference from another RFID interrogator includes:

waiting for the second half of the synchronization pulse;

if a pulse is detected in the second half of the synchronization pulse, determining if another pulse was detected in the second half of a synchronization pulse transmitted by the first RFID interrogator immediately before the synchronization pulse; and

if so, determining that an interference with the second RFID interrogator is detected.

13. The method ofclaim 11, wherein if no interference is detected, the method further comprises processing any RFID tag transmission received during the second half of the synchronization pulse.

14. The method ofclaim 11, wherein synchronizing the transmission of the next synchronization pulse of the first RFID interrogator with the next synchronization pulse of the second RFID interrogator includes delaying the transmission of the next synchronization pulse of the first RFID interrogator so that the start of the next synchronization pulse of the first RFID interrogator coincides with the start of the next synchronization pulse of the second RFID interrogator.

15. The method ofclaim 11, wherein waiting for the expected data from the nearby RFID tag includes waiting for a random number of synchronization pulse periods.

16. The method ofclaim 11, wherein the first and the second RFID interrogators are wireless coupled to each other.

17. The method ofclaim 11, wherein the second RFID interrogator moves into a wireless detection range of the first RFID interrogator.

18. The method ofclaim 11, wherein at least one of the first RFID interrogator and the second RFID interrogator is a mobile RFID interrogator.

19. The method ofclaim 11, wherein the first RFID interrogator is a mobile RFID interrogator mounted on a forklift for warehouse tracking.

20. The method ofclaim 11, wherein the first RFID interrogator is used as a hand-held scanner for shipment tracking.

Images